Martijn van Steenbergen wrote:
Hello,
Suppose I have two projects: 1) one that defines a monad transformer and
an accompanying type class that captures my monad-specific operations
and 2) one that uses the other project, combining the monad transformer
with, say, Parsec.
Now while writing my Parsec parser I want to use my monad transformer
operations without using lift: I need an instance MyMonadT Parsec. Where
should this instance go? I can think of three answers, all unsatisfactory:
1) For obvious reasons it shouldn't go in the Parsec package.
2) For pretty much the same reasons it shouldn't go in my monad
transformer package, either. Also, it is undesirable to add a dependency
on Parsec just for this instance, and the package should not have to
know about the projects that are going to use it.
3) If I put it in the second project it is an orphan instance, which is
undesirable for well-known reasons.
What is the best solution?
4) Define a newtype of MyMonadT Parsec and declare instances of MyMonad
and Parsec for it.
Yes, I know Parsec is (an alias for) a data type, not a type class. But
for the general problem, using newtype wrappers is often the best
solution when it's possible. This is one of the reasons why it's good to
define type classes for specialty monads rather than hard-wiring the
types of functions to use the one concrete instance. (For instance, this
is one of the things that rocks about the LogicT library; you can add a
StateT on top of a MonadLogic and it all works great.)
For the actual case of Parsec, you could try defining a GenParser class
and giving it all the combinators as methods, but that's a good deal of
work and may not scale to your task.
--
Live well,
~wren
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